Hi All, thanks for the replies! I'm still learning about some of this stuff. The system is 6V. Choke seems to work OK. I believe the
carb is original, I did have that cleaned and tuned by a pro. The
coil looks like an easy and inexpensive thing to replace so I will try it and report the results. I'm going to have to wait a few weeks to try it though, after return from vacation. Strangest thing about this problem is the bipolar behavior - sometimes starts and runs fine, sometimes starts and shows this low, rough idle which doesn't clear up even as the engines warms.
The ignition
coil is an amazing little pulse transformer that multiplies low voltage received from the battery or alternator to many thousands of volts when the breaker points open and close.
The
coil contains a primary winding, a slot iron core, and a secondary winding. The high voltage output of the
coil is directed to the appropriate spark plug by the distributor. Without this high voltage, there is no spark and, consequently, internal combustion cannot take place within the engine. The bottom line, simply put, is this: without spark, the engine doesn’t run. So if your T-Bird keeps cranking but won’t start, a bad
coil may be the culprit.
After making sure that fuel is entering the engine, the next thing to check is that there is spark to ignite the fuel. Checking for spark is pretty easy to do. You simply pull a spark plug from the engine (or use a spare plug if you have one in your
parts box, snap one of the plug wires onto it and make sure the shank of the plug is grounded against the engine block. Do not hold onto the spark plug with your bare hands unless you want to get a really nasty jolt. Spark plug boot pulling pliers or other insulated-handle pliers are a good thing to use for this purpose. Have someone crank the engine and observe the center electrode of the plug. If there’s sufficient voltage reaching the plug, you should see a bright blue spark jump across the gap of the electrode every time that cylinder fires. If you see no spark, the
coil is a good thing to suspect next.
You’ll need a digital
multimeter to check the resistance levels of your
coil. The side terminals of the
coil are marked positive (+) and negative (-) and these are where you can measure the resistance of the primary windings. Set the
multimeter to the 200-ohm setting and attach the meter’s leads corresponding to the terminal markings, red being positive and black being negative. The typical resistance reading on the primary side is .3 to 2.0 ohms.
Next, you’ll want to measure the resistance of the secondary
coil, and this is the real business-end of the
coil. Switch the meter’s resistance range to the 20K-ohm setting and leave the negative (black) meter lead connected to the distributor negative post on the
coil. Connect the red meter lead to the center terminal of the
coil. The reading here should be 6 to 30K. If your
coil reads under 6K, then chances are pretty good that this is the reason you’re not getting any spark or a very weak one. The
coil should also be checked hot to insure that the circuit is not breaking down when it reaches operating temperature.
doug7740
1955 Thunderbird Blue
